The present invention is generally directed to stencil masks used in the formation of electrically conductive lines on ceramic substrate materials. More particularly, the present invention is directed to a mesh pattern for a stencil mask which is particularly useful for deposition of conductive paste through the mask and onto a ceramic substrate for conductor patterning. Even more particularly, the present invention is directed to a method which facilitates automatic generation for stencil mesh patterns.
In the fabrication of packages for electronic circuit chips it is necessary to deposit finely detailed patterns of conductive material on insulative substrates such as ceramic materials. In order to provide these conductive patterns conductive paste is deposited on the insulative substrate. The conductive paste or other appropriate material provides the basis for further processing steps which provide the substrate with electrically conductive power and signal patterns. However, it is noted that this subsequent processing does not form any part of the present invention. Rather, the present invention is directed to a stencil mask through which various materials are deposited onto an insulated substrate which is typically a ceramic material. The stencil masks of the present invention are typically metal and are preferably made out of a material such as molybdenum.
These metal screening stencil masks are preferably produced by a subtractive process. A molybdenum foil is coated with a positive photoresist and then imaged. The exposed molybdenum is then etched to form the "through" features. The process is then completed by stripping off the photoresist. These masks have two sides. A first side is a "design" side which contains the pattern of conductive paste which is intended for placement on an insulative substrate such as a ceramic greensheet. Such greensheets are employed in high end packaging for multi-chip modules and in thermal conduction modules used in high end computer systems.
The other side of the mask is the so-called mesh side. The mesh side is used to hold and support the mask design side structure during the screening cycle. Certain mask designs are only possible using mesh structures. These include square hatches, cross hatches and stencils masks for power planes.
As indicated above, the patterning on one side of a stencil mask is in fact different from the patterning on the other side. There is thus a "design" side and a "mesh" side. Furthermore, even apart from the differences in two-dimensional patterning which exist on different sides of the stencil mask, there are other differences that exist between the design side and the mesh side. In particular, etching of the design side produces apertures having different profiles when viewed from the mesh side of the mask as opposed to profiles as seen from the opposite or design side of the stencil mask. In particular, the mesh patterning provides significant structural integrity to a stencil mask. In particular, as indicated certain mask designs require the use of mesh structures to insure both structural integrity and the durability of the stencil mask over many application cycles.
In particular stencil masks for square hatching are used to produce fixed layers in certain semi-conductor packaging arrangement. These design layers are typically used to distribute power and to reduce noise inside the packaging configuration. The pattern comprises a series of lines in horizontal and vertical direction intersecting each other at substantially 90.degree. angles. The line spacing is one design grid wide on so called full dense square hatches (FDSH). On half dense square hatches (HDSH) the pattern is two design grids wide. In particular, in these mask designs, through-vias pass in this layer and are formed by isolating the via pattern with metal on the mask thus not allowing the square between two horizontal and vertical lines be filled with paste.